1. Field of the Invention
This invention relates to a data storage system for an earth station and more particularly to the storage of data representing the attitude and spin rate of the spacecraft.
2. Description of the Prior Art
During all phases of satellite operation including the transfer orbit and on-orbit maneuvers such as station keeping, data from the spacecraft is examined at ground stations to determine if the spacecraft is properly performing in its intended mission. Among the data examined are the output of various sensors located on the satellite, relating to spacecraft attitude and spin rate. The output of these sensors, which are usually in the form of pulses, are transmitted from the satellite to a receiver located at a ground station. The equipment located at the ground station must establish the time relationship between the leading and trailing edges of the various sensor pulses so that a programmed digital computer may be used to calculate spacecraft attitude and spin rate. The attitude is determined by the computer from the times of occurrence of the leading and trailing edges of the pulses developed by a suitable attitude sensor, such as a horizon sensor, located on the spacecraft. Spacecraft spin rate is determined by computing the time of occurrence between successive pulses from a sensor, such as a sun sensor, suitably affixed to a spinning portion of the spacecraft.
Therefore, the times of occurrence of the leading and trailing edges of the attitude sensor pulse(s) and the leading edge of the spin sensor pulse must be stored in appropriate storage devices located at the ground station.
Conventional design procedures dictate the use of individual counters and storage devices (shift registers) associated with each leading or trailing edge. In certain applications, however, the size, space, and power limitations imposed on the ground station equipment configuration may require that the system be more compact than conventional design of the equipment would provide.
In addition thereto as the time required for computer calculation of spacecraft spin rate and attitude is approximately equal to the time between spin sensor pulses, sufficient storage must be provided at the ground station to prevent the loss of data.